Hoshizuki Kotarou is 26 years old, the school nurse and heir of the school chairman. He often has Tsukiko cleaning his messy office for him and making him "nasty tea". His father was the president of the board and when he retired he gave the position to his sister, Koharu. However his Koharu and family decide they all want to go work overseas so Kotarou ends up taking over the position. He's very weak to alcohol and falls asleep quickly if he drinks so he tries to avoid it. During the school festival he tells Tsukiko not to fall in love with him because he would never be able to love her back.

Kotarou wanted to become a doctor but he gave up his dream because he couldn't save Iku's dying sister. Yui had feelings for him, and Kotarou didn't know what to do because he only saw her and Iku as "younger siblings" so he ended up avoiding her, ever since then he was in deep regret and blamed himself for not listening to her.

After the school trip, Kotarou realizes he can't hold his feelings in for Tsukiko anymore so he agrees to go to an omiai that Koharu's been forcing him for a while. Tsukiko is upset and tells him her feelings and says she doesn't want him to go. He rejects her and goes anyway. She ends up waiting for him by the school gates and it begins to rain.

She ends up waiting until Kotarou arrives. When he does she passes out from the cold, He takes her to her room and basically there they end up admitting their feelings to each other and her telling Kotarou that Yui wanted him to be happy. Kotarou and Tsukiko end up living together after she graduates college.